I think once you become used to the idea of writing fanfic, crossovers become inevitable. If you can play with one universe's characters, why not two or more at once? Even if the fit's a little uneasy, it's well worth it, if only for the goofy pleasure of juxtaposition.

It reminds me of when I was little and would throw my Strawberry Shortcake and Hollie Hobbie dolls in with my Barbies. In the steamy universe of the playroom, anything went. Just because Strawberry and Hollie were 2/3 Barbie's size and technically minors was no reason to keep them from trying to stealing Ken away, if they could. (This being my innocent pre-slashy days, before I realized Ken was really lusting after my lone, inherited GI Joe.)